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Don’t Kill Your Starter Plants: Learn How To Harden Off Plants Correctly

There is nothing more heartbreaking that accidentally killing your starter plants because you did not harden off plants correctly.

What Does Hardening Off Plants Mean?

The main goal in hardening off your starter plants is to get them ready for the real outdoor world. Up until now they have lived a cushy life in a greenhouse type environment, either in your house or an actual greenhouse.

Your starter plants are not yet ready to withstand the full strength of the sun, the power of the wind and wide range of temperatures from day to night.

This is where the process of hardening off your plants comes in. Over several days you will slowly get your plants acclimatized to the outside world.

Always harden off plants before transplanting so you don’t kill the plants you have spent your time and money growing.

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How To Harden Off Plants:

Over about a week or more you will gradually take your starter plants outside to a protected area safe from the full wind, sun and really cool temps.

The first day place your plants in this protected space for only an hour or so at first. This is important as you can shock your plants easily those first couple days.

The next day a little longer and start to introduce short amounts of direct sun.

You will see that the plants grow sturdier as they can get more comfortable with being outside. Eventually you can leave them outside all day and possibly over night if they are a hardier plant.

To harden off plants allow this process to take about a week on average, it could be a little shorter but try not to. It can also be much longer but that can get challenging. Keep reading below as there are a few things to keep en eye out for.

Going slower in introducing them to the outside world just increases your chance of success. (if your anything like me though, you have been waiting for months, and can’t get them in the ground fast enough.)

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Timing The Hardening Off Process With Your Last Frost Date

Ideally you will time this process with your last frost date, the warmer weather and when you have time to transplant so that you only have to worry about this process for a short time.

Potential Plant Damage

You cannot leave your plants completely unattended for really long periods while you are hardening them off. The weather can change quite quickly or they may not be ready for the weather you are subjecting them too.

1JE4AJxnTyyJaIA8uUf0 Hardening off plants

Watch Out For:

  • Keep a close eye on their water needs. 
    • If they are getting lots of sun those little starter plant pots can dry out really quickly! Once I start to leave them out for full days if the sun is strong I often find I need to water twice a day.
  • If you notice the leaves turning brown on the tips anywhere this means the sun is too strong or too soon and move them into the shade or part-shade depending.
  • If it is particularly windy be sure to give them protection on all sides so they don’t get bashed around or worse the stem gets snapped.
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Over the week watch how they react to the different weather conditions as you gradually increase their exposure length. Some plants are quite hardy and others are quite tender. This means they will harden off at different rates.

This also means you can plant the hardier ones sooner in the ground as they can tolerate the colder temperatures (sometimes even frost) at the beginning of the season.

Hardier Cool Temperature Plants

This a short list of plants that can tolerate colder temps and even some frost. Make sure to cover them as seedlings and starter plants are still more delicate. You can harden off plants that like cool temps first.

  • Peas
  • Brassicas (Kale, Kohl Rabi, Broccoli, Brussel Sprouts, Cauliflower, Cabbage, Bak Choi)
  • Onions
  • Potatoes
  • Chard
  • Spinach
  • Lettuce
  • Mustard
  • Alyssum
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Peas almost ready for transplanting

Delicate Heat Loving Plants

These plants it’s better to wait until you have warm nights and days. Aim for at least 10C (50F) nights before transplanting these.

These plants will thank you! Often they will stall when transplanted in colder temperatures. Waiting until it’s warmer will be easier on them and often give you less transplant shock.

  • Tomatoes
  • Peppers
  • Eggplant
  • Basil, Oregano, Thyme, Rosemary, Lavender
  • Sunflowers
  • Cucumbers, Squash, Pumpkins
  • Corn
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Squash starter plants are not a fan of cold weather.

The process to harden off plants is fairly simple as you can see. It only requires a bit of patience and close attention until they are ready to be transplanted in the ground.

Use A Greenhouse For Easier Hardening Off

One of my favourite tools for to harden off plants is my little pop-up greenhouse. It saves me a lot of time moving the plants back and forth from inside the house. I can protect plants from strong wind inside the greenhouse as well as give them shelter over night. I have written a detailed blog post on purchasing a greenhouse. You can find it by clicking this link here.

If you are ready to purchase a small greenhouse then this is one I would recommend. It’s small, easy to set up and a good price to get you started.

You can find it at this link here.

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